Security researchers have scuttled the Virut botnet, which was comprised of an estimated 300,000 compromised zombie systems, by shutting down numerous domain names associated with the botnet.

"NASK, the operator of the Polish domain registry, took over 23 of these domains ... in an effort to protect Internet users from Virut-related threats," said Thomas Morrison, who's part of the Spamhaus project, in an announcement. "Name servers for those domains were changed to sinkhole.cert.pl, controlled by CERT Polska [CERT Poland] -- an incident response team operated by NASK."

The cooperative effort between the anti-spam group Spamhaus and CERT Poland was supported by NASK, which is a Polish research and development organization and data networks operator. NASK also operates the top-level .pl domain in Poland.

According to Spamhaus, Virut was being controlled by command-and-control (C&C) servers that used top-level Austrian, Polish, and Russian domain names. "In cooperation with the Polish CERT and the registrar home.pl, we managed to get all the Virut domain names within the .pl ccTLD sinkholed," said Morrison. "In addition, Spamhaus reached out to the Austrian CERT and the Russian-based Company Group-IB CERT-GIB to shut down the remaining Virut domains within the .at and .ru ccTLDs. In cooperation with Spamhaus, and due to the evidence and intelligence provided by Spamhaus, CERT-GIB was able to shut down all the Virut domains within the .ru ccTLD within a few hours."

Unfortunately, the top-level Austrian domain name registrar hasn't yet pulled the plug on Austrian domains that are being used by the Virut botmasters. "Having alerted both nic.at and the Austrian CERT multiple times about this issue, we hope that they can soon follow the examples set by the work done with .pl and .ru," said Morrison.

Spamhaus is operating in somewhat uncharted legal territory. Notably, officials at NASK said this was the first time that top-level domain names in Poland had been forcibly blocked. "NASK's actions are aimed at protecting Internet users from threats that involved the botnet built with Virut-infected machines, such as DDoS attacks, spam and data theft," read an advisory issued by NASK.

The Virut botnet was first spotted in 2006 and is believed to be controlled by Russian criminals. In the past seven years, however, the Virut botnet -- which offers a pay-per-install service that charges customers based on the number of PCs the botmasters ultimately infect -- has grown substantially larger. According to Kaspersky Lab, in the third quarter of 2012, 5.5% of all viruses it detected on infected PCs globally were Virut, which made the malware the fifth most widely seen malicious code at that time.

Many of the botnet's dozens of command-and-control systems were operating from top-level domain names in Poland. "Among the C&C servers used by W32.Virut, the domains irc.zief.pl and proxim.ircgalaxy.pl are used by the threat in order to receive instructions," read a Virut botnet overview published by Symantec. "However, recent versions have also included a domain generator backup that is used if the hardcoded servers cannot be reached."

Symantec said it noticed a recent change in Virut behavior, when "long-running Virut C&C domains stopped responding to connecting clients around mid-November 2012 and had a corresponding registrar status change." In fact, the two domain names mentioned -- among others -- were changed to read "undergoing proceeding," as in being subject to a judicial proceeding.

When that happened, Symantec said Virut's operators switched to the domain generator backup, which let researchers gain a closer look at the botnet's operations. Based on the data subsequently generated -- after Symantec sinkholed the botnet domains for three days -- it found that an estimated 300,000 infected PCs per day were connecting to Virut's C&C servers.

According to Poland CERT, that daily infection rate was a fraction of the number of PCs just in Poland that were being infected with the malware on an annual basis. The scale of the phenomenon was massive, according to NASK: in 2012 for Poland alone, over 890,000 unique IP addresses were reported to be infected by Virut.

As malware gets increasingly sophisticated, so, too, must the technology and strategies we use to detect and eradicate it (or, better yet, stop it before it ever makes it onto network systems). Our Rooting Out Sophisticated Malware report examines the tools, technologies and strategies that can ease some of the burden. (Free registration required.)

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